Feedforward linearization of RF power amplifiers

ABSTRACT

RF amplifier system ( 200 ) incorporating feedforward linearization. The system includes a digital waveform source ( 202 ) generating digital data s(t) representative of at least one analog signal. The system also includes a feedforward linearization circuit for reducing a distortion of an RF power amplifier ( 212 ). The feedforward linearization circuit includes a differential amplifier ( 230 ) arranged for generating an error signal. The error signal is determined based on a difference between the distorted RF output signal and an analog RF reference signal ( 229 ) generated from the digital data.

BACKGROUND OF THE INVENTION

1. Statement of the Technical Field

The inventive arrangements relate to methods for linearizing RF poweramplifiers, and more particularly to a method for providing an envelopeelimination and restoration (EER) amplifier with enhanced linearity.

2. Description of the Related Art

The migration of broadcast and other communications industries tocomplex digital waveforms has necessitated a degree of amplifierlinearity that is unprecedented. Concurrently, there is a continuingdemand for amplifiers that operate more efficiently and offer reducedpower consumption. In the case of large transmitter installations,greater efficiency is important for reducing waste heat and costs. Inother applications, such as that involving portable transceiverequipment, efficiency is important for reducing size, weight, andbattery consumption.

One type of RF power amplifier which offers improved efficiency is theenvelope elimination and restoration (EER) amplifiers. EER amplifiersare well known in the art and can achieve very highly efficientconversion of DC energy to RF energy for complex waveforms having avarying envelope. They operate by separately processing the envelope andphase information contained in a modulated input signal. The phaseinformation is communicated to a power amplifier where it is amplifiedas a constant envelope signal. This permits such phase information to beamplified using highly efficient non-linear amplifiers. The envelopeinformation contained in the input signal is restored to the phaseinformation after the signal has been amplified.

Although highly efficient, EER amplifiers using Class E topologies areknown to have poor linearity. This poor linearity causes significantamounts of signal distortion. For example, such distortion often arisesfrom pulse-width modulator circuits that are used to control the outputenvelope voltage, and from switching non-linearities which exist in thecircuit used for amplifying the phase information. The nonlinearitiescause spectral re-growth (out-of-band noise), which leads to adjacentchannel interference. It also causes in-band distortion, which degradesthe bit-error rate (BER) performance for digital modulation waveforms.In order to comply with FCC spectral masks, reduce BER, and achieveacceptable amplifier efficiency, linearization is necessary.

Distortion associated with RF power amplifiers is often characterized bymeans of an amplitude to amplitude (AM-to-AM) modulation curve and anamplitude-to-phase (AM-to-PM) modulation curve. The AM-to-AM modulationcurve shows the RF power amplifier gain as a function of the inputpower. The AM-to-PM modulation curve shows the output phase variation ofthe RF power amplifier as a function of the input power. It will beappreciated that AM-to-AM distortion and AM-to-PM distortion canadversely affect the performance of an RF communication system. Forexample, such distortion can make it difficult to recover symbols at areceiving end of a communication link.

One well known method for improving the linearity of RF power amplifiersis known as feedforward linearization. With feedforward linearization,an RF splitter is typically used to separate a source signal into twoseparate signals. These two signals include a amplifier input signal anda reference signal. The amplifier input signal is provided to theamplifier as an input. A directional RF coupler is used to obtain asample of the distorted output signal from the RF power amplifier. Thereference signal and the sampled output from the directional coupler arecommunicated to separate inputs of a 180° hybrid RF signal combiner. The180° hybrid RF hybrid combiner subtracts the reference signal from thedistorted amplifier output. The resulting output from the combiner is anerror signal. The error signal is subsequently amplified so as to scalethe error signal to equal the power level of any distortion contained inthe distorted output signal from the RF power amplifier. The errorsignal is then subtracted from the distorted output signal of the RFpower amplifier to remove the distortion from the output signal.

Feedforward linearization is effective at improving amplifier linearity.However, it has not been particularly practical for certain amplifierapplications. For example, the relatively large magnitude of the errorsignal needed to improve the linearity of highly non-linear amplifierscan require a relatively high power RF amplifier for scaling the errorsignal. The necessity for such a relatively high power RF amplifier forscaling the error signal can reduce the overall efficiency of theamplifier system. Thus, feedforward linearization has been limited withregard to its usefulness as applied to highly non-linear amplifiers,such as the EER type amplifier.

Another limitation of feedforward linearization concerns bandwidth. Infeedforward linearization systems, it is important for the error signalto be a highly accurate representation of the actual distortion producedby the RF power amplifier. A distorted error signal will not properlyremove non-linearities from the output of the amplifier. However, in thecase where the signals to be amplified are wideband RF signalsinaccuracy of the error signal can occur. For example, such inaccuraciescan result from amplitude and phase variations which exist across theoperating bandwidth of the RF components used to form and process theerror signal. As noted above, such RF components can include RF signalsplitters and 180° RF hybrid combiner circuits.

SUMMARY OF THE INVENTION

The invention concerns an RF amplifier system incorporating feedforwardlinearization. The system includes a digital multiplexer coupled to adigital waveform source. The digital multiplexer is configured togenerate first and second instances of the digital data. A first dataconverting subsystem is coupled to the digital multiplexer forconverting the first instance of the digital data to analog magnitudeand phase signals defining the analog signal. An RF amplifier is coupledto the first data converting subsystem and is responsive to themagnitude and phase signals for generating a distorted RF output signalmodulated by one or more of the magnitude and phase signals. A seconddata converting subsystem is configured for receiving the secondinstance of the digital data from the digital multiplexer and convertingthe second instance of the digital data to an analog RF referencesignal.

A feedforward linearization circuit is provided for reducing adistortion of the RF amplifier. The distorted RF output signal and theanalog RF reference signal are communicated to the feedforwardlinearization circuit. The feedforward linearization circuit includes adifferential amplifier arranged for generating an error signalrepresenting a difference between the distorted RF output signal and theanalog RF reference signal. The feedforward linearization circuit alsoincludes a combiner for combining the error signal with the distorted RFoutput signal for removing a distorted portion of the distorted RFoutput signal.

The RF amplifier system advantageously includes a digital data timedelay device coupled to the digital multiplexer. The digital time delaydevice is configured for selectively delaying the second instance of thedigital data so that the distorted RF output signal and the analog RFreference signal can be time aligned when they are communicated to thedifferential amplifier.

The invention also includes a method for linearizing an output signal ofan RF amplifier. The method includes the steps of generating first andsecond instances of a digital data s(t) using a digital multiplexer. Themethod also includes converting the first instance of the digital datato analog magnitude and phase signals. A distorted RF output signal isgenerated by an RF amplifier responsive to the magnitude and phasesignals. The RF output signal is modulated by at least one of themagnitude and phase signals. The method further includes converting thesecond instance of the digital data to an analog RF reference signal.The method continues by reducing a distortion of the distorted RF outputsignal using a feedforward linearization circuit. An error signal isgenerated by the feedforward linearization circuit. The error signalrepresents a difference between the distorted RF output signal and theanalog RF reference signal. The method continues with the step ofcombining the error signal with the distorted RF output signal forremoving a distorted portion of the distorted RF output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described with reference to the following drawingfigures, in which like numerals represent like items throughout thefigures, and in which:

FIG. 1 is a block diagram of a conventional RF power amplifierarrangement which incorporates a feedforward system for improving thelinearity of the RF power amplifier.

FIG. 2 is a block diagram that is useful for understanding anarrangement for an RF power amplifier arrangement which incorporates animproved feedforward system for correcting non-linearities in an outputof an RF power amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described more fully hereinafter withreference to accompanying drawings, in which illustrative embodiments ofthe invention are shown. This invention, may however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. For example, the present invention can beembodied as a method, a data processing system, or a computer programproduct. Accordingly, the present invention can take the form as anentirely hardware embodiment, an entirely software embodiment, or ahardware/software embodiment.

The system described herein is intended for improving the linearity ofan RF power amplifier, and more particularly for improving the linearityof RF power amplifiers used to amplify broadband signals. As usedherein, the term linear and/or linearity is used to describe the extentto which an RF amplifier is able to produce an amplified output signalwhich has a amplitude which is related to its input signal by someconstant scaling factor over a defined dynamic operating range of the RFamplifier. Similarly, with regard to phase, the term linear or linearityis used to describe the degree to which such an RF amplifier can producean amplified output signal which has a phase which is related to itsinput signal by some constant value over a defined dynamic operatingrange of the RF amplifier. The dynamic operating range, as that term isused herein, includes an expected range of signal amplitudes, andanticipated signal bandwidth.

There is a continuing demand for amplifiers that operate moreefficiently with unprecedented levels of linearity. Certain types ofamplifiers, such as envelope elimination and restoration (EER)amplifiers, are known to have very high operating efficiency. However,these same amplifiers are also known to have poor linearity. Accordingto an embodiment of the invention, the linearity of such amplifiers canbe improved by using an improved feedforward linearization technique.

Referring now to FIG. 1, there is shown a simplified block diagram of anRF power amplifier system 100 which incorporates a feedforward systemfor improving the amplifier's linearity. In the RF power amplifiersystem 100, a waveform source 102 communicates a source RF signal to anRF power divider 104. The RF power divider is conventionally used tosplit an RF source signal into two separate signals. Typically, each ofthese signals will have an RF power which is approximately half that ofthe source RF signal. The two signals thus produced include an amplifierinput signal and a reference signal. The amplifier input signal isprovided as an input to the RF power amplifier 106. A directionalcoupler 108 is conventionally used to obtain a low power sample of thedistorted output signal from the RF power amplifier 106.

The reference signal and the sampled output signal from the directionalcoupler 108 are communicated to separate inputs of a 180° hybrid RFsignal combiner 114. The conventional 180° hybrid RF signal combinersubtracts the reference signal from the distorted amplifier outputsignal. The resulting output from the 180° hybrid RF signal combiner 114is an error signal. The error signal is subsequently amplified in alinear error amplifier 116 so as to scale the error signal. Inparticular, the error signal is amplified so that its power level isproperly scaled to equal the power level of any distortion contained inthe distorted output signal from the RF power amplifier 106. Thedistorted output signal from the directional coupler is alsocommunicated to a second directional coupler 10. In the seconddirectional coupler 110, the error signal is combined with the distortedoutput signal in a subtraction operation. In particular, the RF errorsignal is subtracted from the distorted output signal of the RF poweramplifier to remove the distortion from the output signal.

FIG. 2 shows a simplified block diagram of an RF power amplifier system200 with feedforward linearization incorporating a feedforwardarrangement for removing non-linearities from the amplifier output. Thearrangement shown is particularly useful for RF power amplifiersintended for use with wideband RF signals.

Referring now to FIG. 2, the RF power amplifier system 200 includes adigital waveform source 202 that generates digital data signal s(t). Thedigital data signal s(t) can be a conventional complex source signal. Asused herein, a complex signal is any signal that is represented in termsof real and imaginary signal components. For convenience in describingthe present invention, the digital data signal can be understood to becomprised of digital data that represents an analog signal comprised ofin phase (I) and quadrature (Q) component vectors (I/Q componentsignal). Digital data of this type is well known in the art andtherefore will not be described here in detail. Still, it should beunderstood that the invention is not limited in this regard. Instead,any other digital representation of a complex time varying analog signalcan also be used. Regardless of the particular format selected for s(t)it should be understood that the digital waveform source 202 generates asignal which can be considered ideal in that it is an exact digitalrepresentation of the desired analog signal. In particular the digitaldata signal s(t) lacks any distortion associated with conventionalanalog signal processing.

The digital waveform source 202 can be realized in computer hardware,software, or a combination of hardware and software. In this regard itshould be appreciated that the digital waveform source can be generatedin one digital processing system, or in a distributed fashion wheredifferent processing elements are spread across several interconnectedsystems. Any kind of computer system, or other apparatus adapted forcarrying out the methods described herein, is suited. A typicalcombination of hardware and software could be a general purpose computerprocessor or digital signal processor loaded with a computer programthat controls the computer system such that it generates time varyingdigital representations of the I and Q signal components. Computerprogram or application in the present context means any expression, inany language, code or notation, of a set of instructions intended tocause a system having an information processing capability to perform aparticular function either directly or after either or both of thefollowing a) conversion to another language, code or notation; b)reproduction in a different material form.

Referring again to FIG. 2, the digital data signal s(t) is communicatedto reference signal generator block 203. Reference signal generatorblock 203 includes at least one digital circuit which is capable ofproviding two identical outputs comprising the digital data signal s(t)from a single digital data signal s(t) input. As will be readilyappreciated by those skilled in the art, a wide variety of digital datacircuits can be used for this purpose. For example, a digitalmultiplexer or digital data buffer can be used for this purpose. Forconvenience, the digital circuit is shown to be a digital multiplexer204. However, the invention is not limited in this regard. All that isnecessary is that some means be provided for generating two identicaldigital data signals s(t).

The digital data signal s(t) is used to generate an analog signal whichis used as an input to the amplifier 212. Depending on the type ofamplifier 212 which is used, it may be desirable to convert the digitaldata signal to a different format. A first data conversion subsystem 213is provided for this purpose. For example, EER type amplifiers haveseparate processing paths for phase and amplitude information. For thesetypes of amplifiers, it is necessary to convert an I/Q component signalto two analog signals that respectively represent amplitude and phaseinformation. This amplitude and phase information is communicated to theEER amplifier as an input signal as is well known in the art.

If the inventive arrangements are intended for use with an EER typeamplifier, the digital data signal s(t) is communicated to first dataconversion subsystem 213 which includes a signal format converter 206.In the embodiment shown, the signal format converter would preferably bean I/Q to amplitude/phase (I/Q to A/P) converter. The signal formatconverter 206 converts the digital data signal s(t) (comprised of I andQ components) to an equivalent signal s′(t) in a different format. Theequivalent signal s′(t) in this case is defined by a first componentcomprising a time varying amplitude signal A(t) and a second componentcomprised of a carrier signal that includes a time varying phase angleΦ(t). Converters of this type are well known in the art. Accordingly,signal format converter 206 will not be described in detail herein.

Those skilled in the art will readily appreciate that various types ofRF power amplifiers may require signal formats other than that which isrequired by an EER type amplifier. Accordingly, if the invention isintended to be used with such other types of amplifiers, it may benecessary to substitute a different type of signal format converter inplace of the I/Q to A/P converter. Thus it should be understood thatinvention is not limited to the use of an I/Q to A/P converter. Instead,any other suitable converter can be used for a particular amplifierapplication, and all such alternative converters are intended to beincluded in the scope of the present invention.

It should be understood that the magnitude and phase components A(t) andΦ(t) can be in digital format. Such signals must be converted to ananalog format before being communicated to the RF power amplifier 212.For this purpose, the first data conversion sub-system 213 can alsoinclude digital to analog converters. For example, magnitude and phasecomponents A(t) and Φ(t) are advantageously communicated to digital toanalog (D/A) converters 208, 210. D/A converters are well known in theart and therefore will not be described here in detail.

The analog output from digital to analog converter 208, 210 will becommunicated to power amplifier 212 where the signal will be amplified.According to one embodiment of the invention, the power amplifier 212can be a switching amplifier, which is also sometimes referred to as aClass D amplifier. In power amplifier 212, the “magnitude” input is abaseband analog signal. The RF signal to be transmitted is generated inthe power amplifier 212 and its amplitude modulation is controlled bythe magnitude input signal. The “phase” input signal to power amplifier212 controls the phase modulation of the RF signal that is generated bythe power amplifier 212. Although switching amplifiers as describedherein have several advantages, non-linearities which exist in suchpower amplifiers will result in an output that can be significantlydistorted. Such non-linearities can be particularly significant if thepower amplifier is of the EER type.

The distorted RF output signal from power amplifier 212 will becommunicated to a directional coupler 218. Directional coupler 218includes an input port connected to the power amplifier 212, atransmitted port connected to a delay line 220, an isolated portconnected to a termination resistor 224, and a coupled port connected tothe differential amplifier 230. The directional coupler 218 willcommunicate most of the distorted RF output signal from power amplifier212 to a time delay device 220.

In general, it is preferable that at least about 90% of the distorted RFoutput signal will be communicated to the time delay device 220.According to one embodiment, directional coupler 218 can be selected tobe a 50 dB directional coupler. With such a coupler, the distorted RFoutput signal communicated to the time delay device 220 will generallybe no more than about 1 dB below the power level of distorted outputsignal produced by the power amplifier 212. Still, it should beunderstood that the invention is not limited in this regard. Higher orlower power levels can be communicated to the differential amplifier 230and the time delay device 220.

Time delay device 220 compensates for the time-delays throughdifferential amplifier 230, time delay device 232, and error amplifier234. The time delay device 220 can be any device capable of producing atime delay in signals traversing through the time delay device. Forexample, the time delay device 232 can be a simple RF delay lineconsisting of a length of RF transmission line. Time delay device 220can also have a time-delay control circuit (not shown) which isresponsive to a time delay control signal 221 for varying a time delayproduced by time delay device 220. Such time delay control circuit canbe in place of or in addition to a time delay control circuit (notshown) provided in time delay 232, which is responsive to a time delaycontrol signal 233. The output from the time delay device 220 will becommunicated to a directional coupler 222. Time delay control signals215, 221, and 233 can be generated by an alignment processor 235 whichmonitors one or more signals to ensure timing alignment of the signalscomprising the linearized output from coupler 222. Alternatively,appropriate time delay control signals 215, 221, 233 can be determinedby manual or automated means in an initial alignment process, andthereafter stored in a memory device.

Referring again to directional coupler 218, it can be observed in FIG. 2that a portion of the distorted RF output signal from power amplifier212 is coupled to the negative input of the differential amplifier 230.This signal shall be referred to herein as coupled signal 219. Thecoupled signal 219 will typically have a power level that issignificantly reduced relative to the total RF input power communicatedto the coupler 218 from the power amplifier 212. The actual power levelof the coupled signal 219 will depend on a variety of design factors.However, the power level of coupled signal 219 will typically be no morethan about 10% of the power level of the distorted output signalproduced by the power amplifier 212. For example, the directionalcoupler 218 can be a 50 dB type directional coupler so that the coupledsignal will have a power level that is about 50 dB below the power levelof the distorted RF output signal from the power amplifier 212. Still,it should be understood that the invention is not limited in thisregard.

A reference signal 229 is communicated to the positive input of thedifferential amplifier 230. This reference signal 229 is produced inreference signal generator 203, which will now be described in detail.The digital data signal s(t) provided from digital multiplexer 204 iscommunicated to a digital data time delay device 205 that is suitablefor selectively delaying digital signals. In this regard, it should beunderstood that the digital data time delay device preferably operatesin the digital domain. The digital data time delay device 205 can be ofthe fixed delay type. However, the digital time delay device 205 canalso include a time delay control circuit (not shown) which allowsselective variable control of the amount of delay applied to digitaldata signal s(t). The time delay control circuit can be controlled by atime delay control signal 215 as shown.

The digital time delay device 205 provides a time delay sufficient toensure that the reference signal 229 is properly aligned in time withthe coupled signal 219. This means that the path delays from the digitalmultiplexer 204 to the differential amplifier 230 are equalized for thereference signal 229 and the coupled signal 219. In particular, coupledsignal 219 is delayed by the signal format converter 206, D/A converters208, 210 and the power amplifier 212. In contrast, the reference signalis delayed by the D/A converters 207, 209, and an RF modulator 211. Therespective amounts of delay encountered by each signal can be different.Accordingly, in order to properly compare the coupled signal 219 to thereference signal 229, the signals must be aligned in time. This timealignment is performed by delay device 205.

After the digital signal s(t) is processed by digital data time delaydevice 205, its output is communicated to one or more D/A converters207, 209 which transforms the digital s(t) signal into an analogbaseband signal. For example, these signals can be analog baseband I andQ signals. These signals are subsequently communicated to the RFmodulator 211 which converts these analog baseband signals to an analogRF reference signal 229. For convenience, the combination of D/Aconverters 207, 209 and the RF modulator 211 are referred to herein asthe second data converting subsystem. Alternatively, RF modulator 211can be implemented in the digital-domain, provided that the D/Aconverters can sample at twice the RF frequency. If the RF modulator isimplemented in the digital-domain, then only one D/A is required.

The analog RF reference signal 229 is an ideal reference signal in thesense that it is produced by an RF modulator 211 based on an exactdigital representation of the desired source signal s(t) from digitalwaveform source 202. Also, it has been digitally delayed so as toprovide correct time alignment. As such, it is absent of any significantdistortion, such as the distortion caused by power amplifier 212.

The power level of the coupled signal 219 and the analog RF referencesignal 229 are preferably selected so that they are equal. For example,if the power amplifier 212 has an output power of +53 dBm and thedirectional coupler is a 50 dB directional coupler, then the coupledsignal 219 will have a power level of +3 dBm. In this case, the analogRF reference signal 229 would also be selected to have a power level of+3 dBm. Of course, other power levels can also be used, but it isadvantageous that the power level of the analog RF reference signal 229and the coupled signal power level 219 are equal. When the power levelsare arranged in this way, the output of the differential amplifier 230will be an inverted error signal that represents the distortionintroduced to the power amplifier 212.

The inverted error signal output from the differential amplifier 230 iscommunicated to a time delay device 232. For example, the time delaydevice 232 can be a fixed RF delay line consisting of a length oftransmission line. However, according to a preferred embodiment of theinvention, the time delay device 232 can be selectively variable. Forexample, a variable length transmission line can be used for thispurpose. However, other types of delay lines are also possible and theinvention is not intended to be limited in this regard. For example anyone of a variety of commercially available variable analog delay linedevice can be used. In this regard, it should be understood that thetime delay device 232 can include at least one time delay controlcircuit for selectively varying the amount of time delay applied by thetime delay device 232. As shown in FIG. 2, a time delay control signal233 can be provided as an input to the time delay device to selectivelyvary the time delay. Notably, it is not necessary to have time delaydevice 232. Advantageously, time delay device 232 allows the time-delaycontrol to operate on lower-power signals, while the fixed time-delay220 operates on the high-power signals. Alternatively, time delay device232 can be removed and time-delay control provided for time-delay 220.Removing the time-delay device 232 allows less time-delay to be requiredby device 220.

The inverted error signal output of the delay device 232 is communicatedto at least one error amplifier 234. Error amplifier 234 is a linearamplifier which linearly amplifies the inverted error signal. Accordingto an embodiment of the invention, a gain of the error amplifier can beadjustable by means of a gain adjustment control signal. The erroramplifier 234 advantageously increases the power level of the invertederror signal so that it is equal to the power level of the distortioncontained in the distorted amplifier output signal communicated to thedirectional coupler 222. After amplification, the inverted error signalis communicated from an output of the error amplifier 234 to the inputof directional coupler 222. As noted above, directional couplers arewell known in the art. According to one embodiment, the directionalcoupler 222 can be selected to be a 15 dB directional coupler. In thisregard, it will be appreciated that a 15 dB coupling ratio fordirectional coupler 222 results in a minimal amount of RF power frompower amplifier 212 being communicated to the termination resistor 226.However, the coupling is adequate for providing a sufficient amount ofpower from the error amplifier 234 to the linearized output signal fromdirectional coupler 222 so as to substantially reduce distortion. Still,it should be understood that the particular coupling ratio fordirectional coupler 222 can be chosen by the designer. Of course, theoutput power from error amplifier 234 should be chosen to ensure thatthe error signal produced by the error amplifier has sufficientmagnitude to compensate for distortion contained in the output signalfrom the power amplifier 212.

From the foregoing description it will be understood that there are twosignals that are provided to the directional coupler 222. One signal isthe distorted output signal from the power amplifier 212 and the othersignal is the amplified inverted error signal from error amplifier 234.The directional coupler 222 is a four port device which couples theinverted error signal from error amplifier 234 to the distorted outputsignal. In this regard, it should be understood that the directionalcoupler 222 produces a linearized output signal at an output port thatis the sum of the distorted output signal and the inverted error signal.Since the error signal is inverted, the summing operation can be thoughtof as an operation which involves subtracting the error signal from thedistorted output signal. Since the inverted error signal is an invertedrepresentation of the distortion which is present in the distortedoutput signal, this subtracting operation removes the distortion that ispresent in the distorted output signal. The result is a linearizedoutput.

The present invention includes several features which together representan important departure from the conventional feedforward type amplifiersystem of the prior as shown in FIG. 1. One such feature concerns theway in which the reference signal 229 is processed. Conventionalfeedforward type amplifier systems similar to the one shown in FIG. 1use an analog RF signal splitter 104 to generate a reference signal 229.However, when applied to a feedforward linearization system, suchconventional analog RF signal splitters 104 have several undesirablecharacteristics. Most significantly, conventional analog RF signalsplitters can exhibit frequency dependent phase and amplitudevariations. Stated differently, this means that the transfercharacteristic of the signal splitter 104 can introduce phase andamplitude variations over a bandwidth of an input signal. When used in afeedforward linearization application, such RF signal splitters cancause reference signal variations in phase and amplitude. Consequently,a reference signal will result which is not ideal. Accordingly, theerror signal in such cases will be inaccurate, such that the errorsignal can fail to properly improve the linearity of the output signalfrom amplifier.

In contrast, the present invention forms the reference signal by“splitting” the digital input signal s(t) while the signal is in thedigital domain. This function is performed in digital multiplexer 204.The advantage of this approach is that it avoids the problem of phaseand amplitude variations which typically occur over the bandwidth of anamplifier input signal when a conventional RF splitter is used. Suchvariations can be particularly problematic when the input signal is awideband signal.

Another advantage of the approach described herein is that itfacilitates use of a digital data time delay device 205 which operatesin the digital domain. Use of a digital data time delay device 205 inthis feedforward linearization application is advantageous for severalreasons. The digital data delay device 205 operates in the digitaldomain and therefore avoids any potential distortion of the referencesignal which might otherwise occur if an analog delay device was used.Second, variable time delay devices in the digital domain are relativelyinexpensive, more easily implemented and digitally controlled. This isan advantage over variable analog time delay devices for RF signals. Inparticular, the use of a digital data time delay device 205 means thatthe linearization system can be rapidly reconfigured for use withdifferent types of amplifiers 212.

Different amplifiers 212 can have different time delays associatedtherewith. Use of a digital data delay device 205 for the referencesignal means that the linearization system in FIG. 2 can be used withtwo or more amplifiers without any significant design modifications.Instead, conventional digital control signals can be communicated to thedigital delay device 205 to control the time delay as needed for aparticular application. For example, such control signals can beprovided by a suitable integrated circuit controller or programmableROM.

Another important feature of the present invention concerns the mannerin which the coupled signal 219 is subtracted from the reference signal229. In particular, the present invention makes use of a differentialamplifier 230 in place of a conventional 180° hybrid coupler 114 asshown in FIG. 1. Conventional 180° hybrid couplers suffer from twopotential problems. First, 180° hybrid couples can and do exhibitfrequency dependent variations in phase and amplitude. Such variationscan be particularly problematic when the input signals to the 180°hybrid coupler is used to process signals having a relatively widebandwidth. It will be appreciated by those skilled in the art thatfrequency dependent variations in phase and amplitude can producedistortion which will result in an inverted error signal which isinaccurate. In such case, the error signal can fail to properly improvethe linearity of the output signal from power amplifier 212 because theerror signal does not accurately represent the error.

In contrast, commercially available high precision integrated circuitdifferential amplifiers can offer much improved performance. Currentcommercially available high precision integrated circuit differentialamplifiers are designed to operate at high frequencies. Such amplifiershave dramatically improved linearity as compared to 180° hybridcouplers, particularly when the input signals are of relatively widebandwidths. Accordingly, such differential amplifier can provide a moreaccurate representation of an error signal.

Another problem with 180° hybrid couplers concerns the common moderejection ratio (CMRR). The common-mode rejection ratio (CMRR) of adevice is a measure of that device's tendency to reject input signalscommon to both inputs. In the present case, the CMRR refers to theability of a 180° hybrid coupler 114 (or the differential amplifier 230)to completely and accurately determine the difference between areference signal and a distorted amplifier output signal. A high CMRR isimportant in feedforward linearization applications because thedistortion existing in the distorted amplifier output signal can oftenbe a relatively small voltage compared to the amplitude of the distortedamplifier output signal. Typically, it is difficult to achieve a designof a 180° hybrid coupler with a CMRR of greater than about 50 dB. Incontrast, current commercially available differential amplifierintegrated circuits can achieve a CMRR of 100 dB or better. Use of suchan integrated circuit has now become possible for many RF applicationsbecause of the wide operating bandwidth of currently availabledifferential amplifiers. The combination of these features means that ahigh linearity differential amplifier can be used in place of theconventional 180° hybrid coupler in a feedforward linearization, and canoffer a significant improvement in performance.

In general, the differential amplifier 230 should have a design whichresults in a common mode rejection ratio (CMRR) of at least about 50 dB.The differential amplifier should preferably have a bandwidth that issuitable for a range of input frequencies that are contained in thereference signal 229. For example, if the amplifier system 200 isintended for use with conventional broadcast signals in the AM or FMbroadcast band, a linear differential amplifier having a bandwidth ofbetween 10 MHz and 200 MHz can be used. Notably, the precisespecification for the degree of linearity required of differentialamplifier 230 will depend upon the level of the distortion to beeliminated from power amplifier 212. The differential amplifier 230 mustbe sufficiently free from distortion so that the amplifier canaccurately generate an error signal based on a difference between thereference signal 229 and the coupled signal 219. There are a variety ofcommercially available products that can be used for implementingdifferential amplifier 230.

Notably, the amplifier system described herein can be used for a widevariety of signals, including broadband data signal. However, for wideband data signals, it is important that differential amplifier 230 anderror amplifier 234 have a suitably wide operational bandwidth. In thisregard, it should be understood that the differential amplifier 230 canbe a conventional integrated circuit device, a hybrid circuit design, oran RF type differential amplifier. The particular type of circuitconstruction will be determined by the CMRR and bandwidth capabilityrequired in a particular application for processing particular types ofbroadband signals.

The invention described and claimed herein is not to be limited in scopeby the preferred embodiments herein disclosed, since these embodimentsare intended as illustrations of several aspects of the invention. Anyequivalent embodiments are intended to be within the scope of thisinvention. Indeed, various modifications of the invention in addition tothose shown and described herein will become apparent to those skilledin the art from the foregoing description. Such modifications are alsointended to fall within the scope of the appended claims.

1. An RF amplifier system incorporating feedforward linearization,comprising: a digital multiplexer configured for receiving digital datas(t) from a digital waveform source and for generating first and secondinstances of said digital data; first converting means coupled to saiddigital multiplexer for converting said first instance of said digitaldata to analog magnitude and phase signals defining said analog signal;an RF amplifier coupled to said first converting means and responsive tosaid magnitude and phase signals for generating a distorted RF outputsignal modulated by at least one of said magnitude and phase signals;second converting means configured for receiving said second instance ofsaid digital data from said digital multiplexer and converting saidsecond instance of said digital data to an analog RF reference signal; afeedforward linearization circuit for reducing a distortion of said RFamplifier, said feedforward linearization circuit comprising adifferential amplifier arranged for generating an error signalrepresenting a difference between said distorted RF output signal andsaid analog RF reference signal; and combining means for combining saiderror signal with said distorted RF output signal for removing adistorted portion of said distorted RF output signal.
 2. The RFamplifier system according to claim 1, wherein said digital data is adigital I/Q component signal and said first converting means comprises asignal format converter arranged for converting said digital I/Qcomponent signal to at least one digital data signal comprising phaseand magnitude information.
 3. The RF amplifier system according to claim2, wherein said first converting means further comprises at least onedigital to analog converter coupled to said signal format converter andconfigured for converting said at least one digital data signalcomprising phase and magnitude information to said analog magnitude andphase signals.
 4. The RF amplifier system according to claim 1, furthercomprising a digital data time delay device coupled to said digitalmultiplexer configured for selectively delaying said second instance ofsaid digital data.
 5. The RF amplifier system according to claim 1,wherein said second converting means comprises at least one digital toanalog converter configured for generating analog baseband I and Qcomponents from said second instance of said digital data.
 6. The RFamplifier system according to claim 5, wherein said second conversionmeans further comprises an RF modulator coupled to said at least onedigital to analog converter and responsive to said baseband I and Qcomponents of said analog reference signal for generating said analog RFreference signal.
 7. The RF amplifier according to claim 1, furthercomprising an error amplifier having an input configured for receivingsaid error signal from said differential amplifier and an output coupledto said combining means, said error amplifier configured for amplifyingsaid error signal.
 8. The RF amplifier system according to claim 1,further comprising an analog time delay device coupled to at least oneof said differential amplifier and said error amplifier, said analogtime delay device configured for selectively delaying said error signal.9. The RF amplifier system according to claim 1, wherein said RFamplifier is an EER type amplifier.
 10. A method for linearizing anoutput signal of an RF amplifier, comprising: generating first andsecond instances of a digital data s(t) using a digital multiplexer;converting said first instance of said digital data to analog magnitudeand phase signals; generating a distorted RF output signal modulated byat least one of said magnitude and phase signals using an RF amplifierresponsive to said magnitude and phase signals; converting said secondinstance of said digital data to an analog RF reference signal; reducinga distortion of said distorted RF output signal using a feedforwardlinearization circuit which generates an error signal representing adifference between said distorted RF output signal and said analog RFreference signal, and combines said error signal with said distorted RFoutput signal for removing a distorted portion of said distorted RFoutput signal and for generating at least one linearized output signal,wherein said reducing further comprises monitoring said distorted RFoutput signal and said analog RF reference and generating time delaysignals based on said monitoring for time delaying said distorted RFoutput signal, said analog RF reference signal, and said second instanceof said digital data to ensure a timing alignment for said generating ofsaid linearized output signal.
 11. A method for linearizing an outputsignal of an RF amplifier, comprising: generating first and secondinstances of a digital data s(t) using a digital multiplexer; convertingsaid first instance of said digital data to analog magnitude and phasesignals; generating a distorted RF output signal modulated by at leastone of said magnitude and phase signals using an RF amplifier responsiveto said magnitude and phase signals; converting said second instance ofsaid digital data to an analog RF reference signal; reducing adistortion of said distorted RF output signal using a feedforwardlinearization circuit which generates an error signal representing adifference between said distorted RF output signal and said analog RFreference signal, and combines said error signal with said distorted RFoutput signal for removing a distorted portion of said distorted RFoutput signal, wherein said first instance of said digital data is adigital I/Q component signal and further comprising converting saidfirst instance of said digital data to digital magnitude and phase data.12. The method according to claim 11, further comprising converting saiddigital magnitude and phase data to said analog magnitude and phasesignals.
 13. The method according to claim 10, further comprisingselectively delaying said second instance of said digital data using adigital time delay device.
 14. A method for linearizing an output signalof an RF amplifier, comprising; generating first and second instances ofa digital data s(t) using a digital multiplexer; converting said firstinstance of said digital data to analog magnitude and phase signals;generating a distorted RF output signal modulated by at least one ofsaid magnitude and phase signals using an RF amplifier responsive tosaid magnitude and phase signals; converting said second instance ofsaid digital data to an analog RF reference signal; reducing adistortion of said distorted RF output signal using a feedforwardlinearization circuit which generates an error signal representing adifference between said distorted RF output signal and said analog RFreference signal, and combines said error signal with said distorted RFoutput signal for removing a distorted portion of said distorted RFoutput signal; and generating analog baseband I and Q components fromsaid second instance of said digital data.
 15. The method according toclaim 14, further comprising generating said analog RF reference signalusing an RF modulator responsive to said analog baseband I and Qcomponents.
 16. The RF amplifier according to claim 10, furthercomprising amplifying said error signal using prior to combining saiderror signal with said distorted RF output signal.
 17. The methodaccording to claim 10, further comprising selectively delaying saiderror signal using an analog time delay device.
 18. An RF amplifiersystem incorporating feedforward linearization, comprising: a digitalmultiplexer coupled to a digital waveform source for generating firstand second instances of a digital data signal s(t); first convertingmeans coupled to said digital multiplexer for converting said firstinstance of said digital data to analog magnitude and phase signals; anRF amplifier coupled to said first converting means and generating adistorted RF output signal modulated by said analog magnitude and phasesignals; second converting means configured for receiving said secondinstance of said digital data from said digital multiplexer andconverting said second instance of said digital data to an analog RFreference signal; a feedforward linearization circuit for reducing adistortion of said RF amplifier using an error signal representing adifference between said distorted RF output signal and said analog RFreference signal to generate at least one linearized output signal; andan alignment processor for monitoring said distorted RF output signaland said analog RF reference, and for generating time delay signalsbased on said monitoring for said distorted RF output signal, saidanalog RF reference signal, and said second instance of said digitaldata to ensure a timing alignment for said generating of said linearizedoutput signal.
 19. An RF amplifier system incorporating feedforwardlinearization, comprising: a digital multiplexer coupled to a digitalwaveform source for generating first and second instances of a digitaldata signal s(t); first converting means coupled to said digitalmultiplexer for converting said first instance of said digital data toanalog magnitude and phase signals; an RF amplifier coupled to saidfirst converting means and generating a distorted RF output signalmodulated by said analog magnitude and phase signals; second convertingmeans configured for receiving said second instance of said digital datafrom said digital multiplexer and converting said second instance ofsaid digital data to an analog RF reference signal; and a feedforwardlinearization circuit for reducing a distortion of said RF amplifier,wherein said feedforward linearization circuit is comprised of adifferential amplifier having as a first input a sample of saiddistorted RF output signal, and as a second input said analog RFreference signal, and generating an error signal representing adifference between said distorted RF output signal and said analog RFreference signal.
 20. The RF amplifier system according to claim 19,wherein said feedforward linearization circuit is further comprised ofcombining means configured for combining said error signal with saiddistorted RF output signal for removing a distorted portion of saiddistorted RF output signal.